1. Field of the Invention
The present invention relates to an optical pickup device, and more particularly, to an optical pickup device having a chromatic aberration correction lens to correct a chromatic aberration caused by a change in a wavelength and/or an increase in a wavelength bandwidth of light emitted from a light source, occurring when changing a recording/reproducing power output.
2. Description of the Related Art
The recording capacity of an optical recording and reproducing apparatus is determined by the size S of a light spot formed on an optical disc by the objective lens of an optical pickup device. Generally, the size S of the light spot is proportional to a wavelength λ and is inversely proportional to a numerical aperture (NA). Accordingly, to obtain a higher information recording density than that obtained on conventional optical discs such as CDs or DVDs, an optical pickup device (hereinafter, referred to as a high density optical pickup device) used for next generation DVDs (hereinafter, referred to as HD-DVDs) under development is anticipated to use a light source emitting blue light and an objective lens having a NA of at least 0.6, to reduce the size of the light spot formed on the optical disc.
However, an optical material such as glass or plastic used as the material of the objective lens in the conventional optical pickup device has a very steep change in refractivity in a wavelength band shorter than 650 nm. Table 1 shows changes in refractivity of M-BaCD5N, which is manufactured by Hoya and is used as a glass material for molding the objective lens, according to a wavelength.
TABLE 1Change in refractivity of M-BaCD5N glassChange in wavelengthmanufactured by Hoya650 nm → 651 nm0.000038405 nm → 406 nm0.000154
As seen from Table 1, an optical material has a change in refractivity with respect to a small wavelength change of about 1 nm in a short blue wavelength band, for example, a 405 nm wavelength band, four times larger than in a 650 nm wavelength used in a conventional DVD optical pickup device. Such a steep change in refractivity of the optical material with respect to blue light causes a high density optical recording and reproducing apparatus using a blue light source to be defocused, thereby degrading performance.
In other words, an optical recording and reproducing apparatus uses different recording light power and reproducing light power. This change in the light output power between recording and reproduction causes the wavelength change. For example, in the case of the blue light source, the change in the wavelength is about 0.5-1 nm. Usually, when the output of the light source increases, the wavelength of light emitted from the light source is longer. Accordingly, the high density optical pickup device using blue light has a large chromatic aberration in the objective lens designed for a reference wavelength due to the change in the wavelength during switching between recording light output power and reproducing light output power, causing defocus.
For example, as shown in FIGS. 1 through 3, an objective lens, which has a numerical aperture of 0.65 and is designed for a wavelength of 405 nm, has a large wavefront aberration (also referred to as an optical path difference (OPD)) and defocus with respect to a fine change of about 1 nm in wavelength. FIG. 1 is a graph illustrating intensities of light spots formed on an optical disc according to defocus resulting from a change in light output power between recording and reproduction. FIGS. 2 and 3 are graphs illustrating the amount of the OPD and the amount of defocus, respectively, of the objective lens having a numerical aperture of 0.65, according to the change in the wavelength.
Although defocus caused by the change in the wavelength can be corrected by adjusting the objective lens, it takes a relatively long time to actuate the objective lens using an actuator and to follow the change in the wavelength, and during this time, the quality of a recorded or reproduced signal is degraded. Defocus occurring when output power increases for recording results in a lack of recording light power, and defocus occurring when output power decreases for reproduction increases jitter.
In other words, when the output power of the light source increases when recording information on the optical disc, the wavelength of light emitted from the light source is relatively long, for example, 406 nm, so that the light spot formed on the optical disc is defocused. Until the actuator is adjusted in response to the defocus, recording cannot be performed. Then, when the output power of the light source decreases for reproduction, the wavelength of light emitted from the light source is relatively short, for example, 405 nm. Since the actuator has been adjusted with respect to the lengthened wavelength, the light spot is defocused again. As shown in FIG. 4, the jitter increases in the reproduced signal due to defocus. FIG. 4 is a graph illustrating the amount of jitter in the reproduced signal according to the amount of defocus when the objective lens designed with respect to a reference wavelength of 405 nm and having a numerical aperture of 0.65 is used.
Moreover, when the light source is actuated at a high frequency (HF) to reduce feedback noise of the light source due to light reflected from the optical disc to the light source, a wavelength bandwidth of the light source increases, resulting in chromatic aberration, and this chromatic aberration degrades the reproduced signal.
Accordingly, a high density recordable optical pickup device capable of recording and reproducing repeatedly is required to have an optical system capable of suppressing or correcting chromatic aberration resulting from a change in the wavelength of light emitted from the light source due to the change in output power between recording and reproduction. Japanese Patent Publication No. Hei 9-311271 discloses a structure employing a refraction/diffraction-monolithic-type objective lens to correct chromatic aberration resulting from a change in wavelength. A conventional refraction/diffraction-monolithic-type objective lens is an aspheric lens whose surface receiving or emitting light is aspheric. Diffraction patterns are integrally formed on this aspheric surface so that a refractive lens and a diffraction lens are integrated into a single lens.
The refraction/diffraction-monolithic-type objective lens is designed to satisfy (1+VHOE/V)(n2−1)>0.572 when it is assumed that refractivities of the lens at a central wavelength λ1, a minimum wavelength λ2 and a maximum wavelength λ3 of light emitted from a semiconductor laser are n1, n2 and n3, and that the Abbe numbers of the refractive lens and the diffraction lens are V=(n2−1)/(n1−n3) and VHOE=λ2(λ1-λ3), respectively. Accordingly, the conventional refraction/diffraction-monolithic-type objective lens has a numerical aperture of at least 0.7 and can remove chromatic aberration due to the change in the wavelength of light emitted from the semiconductor laser. However, an optical pickup device employing the conventional refraction/diffraction-monolithic-type objective lens cannot obtain sufficient output power necessary for recording since optical efficiency is lowered to about 70-85% due to the properties of the diffraction lens.